(1) Field of the Invention
This invention relates to an automatic thread supply device for an overlocking sewing machine, to a thread chain which is formed by the overlocking sewing machine when there is no fabric at the sewing area of the overlocking sewing machine, and to a method of forming such a thread chain.
(2) Description of the Prior Art
In the overlocking sewing machine disclosed in U.S. Pat. No. 3,145,672 or the like, a thread chain is formed of entangled needle and looper threads immediately after the rear end of a fabric passes the needle drop point. As disclosed in U.S.Pat. No. 3,123,033, such a thread chain is cut off by a cutter provided in the overlocking sewing machine so that the thread chain extended from the needle drop point has a predetermined length. This thread chain is held and sewn in on the back surface of the leading end of the following fabric.
If the thread chain is expandable, it can be stretched out and sewn in within a stitch width W along a side of a fabric M (FIG. 1a). If it is not expandable, some parts of it shown with T1 and T2 come out of the leading end of the fabric M and the stitch width W (FIG. 1b), resulting in poor appearance.
In order to form an expandable, that is high quality, thread chain, each supply length per stitch of the needle thread and the looper threads should be adjusted. In a conventional overlocking sewing machine, spring pressure of each thread tension regulator is adjusted, namely the tension of each thread is adjusted, when a thread chain is formed. According to this method, however, the supply length ratio of the threads cannot be adjusted accurately enough to form a high quality thread chain.